What we do know is that Naomi Watts has also landed a lead role and that she will be playing a socialite.

Since Naomi's casting was announced yesterday, various theories have popped up online, with some claiming that she will be playing a Targaryen.

While multiple spinoffs were initially in development at HBO, the one penned by Jane Goldman is the only one which has received a pilot order.

According to the official logline, “the series chronicles the world’s descent from the Golden Age of Heroes into its darkest hour."

"And only one thing is for sure: from the horrifying secrets of Westeros’ history to the true origin of the white walkers, the mysteries of the East to the Starks of legend… it’s not the story we think we know.”

Game of Thrones creator George R.R. Martin previously opened up about the spinoff in a blog post.

“None of the characters or actors from Game of Thrones will appear in the new show," Martin said of the prequel in a blog post in June, adding:

"All of the successor shows we’ve been developing have been prequels, as I have mentioned before."

"This one really puts the pre in prequel, since it is set not ninety years before Game of Thrones (like Dunk & Egg), or a few hundred years, but rather ten thousand years (well, assuming the oral histories of the First Men are accurate, but there are maesters at the Citadel who insist it has only been half that long)," he continued. "We’re very early in the process, of course, with the pilot order just in.”

Now that two of the roles have been filled, expect the rest of the cast to be announced in the coming weeks.